Version 3.0.0

[marnusw]

This is a complete rewrite of the component which more closely matches react-addons-css-transition-group. The implementation is more stable and solves some past idiosyncrasies. (#29, #38)

This branch can be installed as version 3.0.0-beta.1

Outstanding items before this branch will be ready to merge:

• Add the option to animate width back in as in Add changeWidth param to allow animating width #31.
• Add an option to prevent a second animation from being initiated while a previous animation is in progress. (This component used to work this way before Improve behavior of transitionLeave=false and height transition #34 was merged.) (This actually doesn't seem like a great idea; if this is needed it seems the parent component should prevent state changes since this component can't just ignore changing children. The parent not knowing whether a transition is active or not won't make this easy, but that is a different problem.)
• When starting a 2nd animation while a previous component is still entering that animation is cancelled and the leave animation starts immediately causing a jerk.